The invention relates to an electromagnetic actuator, and finds particular use in direct current (DC) electromagnetic actuators for actuating engine valves.
Electromagnetic actuators are known that comprise an actuator rod connected to an armature to form a moving assembly associated with a drive member that propels the moving assembly from one extreme position to another, the extreme positions corresponding to the valve being in its open and closed positions.
The drive member comprises at least one coil fitted with a core, one face of the armature being adapted to come into contact with one face of the coil core when the valve is in one of its open or closed positions. The coil serves to move the armature and to retain it so as to hold the moving assembly in the corresponding extreme position. For this purpose, the coil is connected to a DC power supply circuit.
While the actuator is in operation, the coil is powered, thereby creating magnetic flux which is looped via the armature. The coil thus exerts a force of attraction on the armature.
While the armature is still far away from the core of the coil, the total air gap through which the flux needs to pass between the armature and the core is large. The coil then needs to be fed with high current in order to ensure that the force of attraction it develops overcomes the opposing forces to which the moving assembly is subject.
However, as the armature comes close to the core, the total air gap between the armature and the core decreases very quickly, and the power supply electronics is generally not fast enough to reduce the magnitude of the current it is delivering in proportion to the rate at which the air gap is closing.
The attraction force exerted by the coil on the armature at the end of its stroke greatly exceeds the opposing force acting on the moving assembly. The armature thus strikes the core at high speed, thereby producing an undesirable clattering noise, and preventing proper control over the docking speed of the valve against its seat in the closed position.
Attempts have been made to reduce the force of attraction at the end of the stroke by limiting the intensity of the flux that can be established in the armature.
Unfortunately, the armature is generally dimensioned so as to operate at the magnetic saturation limit that corresponds to the maximum flux the coil can develop, as occurs when the armature is remote from the core. Limiting flux does indeed make it possible to reduce the force of attraction at the end of the stroke, but it degrades the performance of the actuator when the armature is remote from the core.
The description below seeks to propose means for adapting the force of attraction of the coil on the armature at the end of its stroke, preferably without degrading the performance of the object of the actuator when the armature is remote from the core.